code: def _get_coord_systems(expr):
      g = preorder_traversal(expr)
      ret = set()
      for i in g:
          if isinstance(i, CoordSys3D):
              ret.add(i)
              g.skip()
      return frozenset(ret)
  
imports: import collections
  from sympy.core.expr import Expr
  from sympy.core import sympify, S, preorder_traversal
  from sympy.vector.coordsysrect import CoordSys3D
  from sympy.vector.vector import Vector, VectorMul, VectorAdd, Cross, Dot
  from sympy.core.function import Derivative
  from sympy.core.add import Add
  from sympy.core.mul import Mul
label_desc: Use image node_id 1 for calling a global function with example usage: _get_coord_systems(expr) and returns: frozenset